The conditions for paralleling 3-phase transformers are same as that required for parallel operation of single-phase transformers with the following additions:
i. The voltage ration must refer to the terminal voltage of primary and secondary.
ii. The phase displacement between primary and secondary voltages must be the same for all transformers which are to be paralleled.
iii. The phase sequence must be the same.
Following points are worthnoting while dealing with 3-phase transformers.
- The calculations are made for one phase only. The value of equivalent impedance used is the equivalent impedance per phase referred to secondary.
- When the impedances of primary and secondary windings are given separately then primary impedance must be transferred to secondary by multiplying it with (transformation ratio)2.
- In case of Y/Δ or Δ/Y transformers the voltage rations as given in the questions, refer to terminal voltages and are quite difference from turn ratio.